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The continuity of antimicrobial resistance development in bacteria changes the type and dose of effective antibiotic treatments and makes routine monitoring studies necessary for successful control of bacterial diseases. This study was aimed to determine the current antibiotic susceptibility of Lactococcus garvieae, which causes significant economic losses in rainbow trout (Oncorhynchus mykiss) farms. In the study, two consecutive visits were made to three farms operating on the banks of a stream during a disease outbreak in the fall of 2018. At each visit, 10 fish showing the signs of disease from each farm (60 fish in total) were sampled and L. garvieae was isolated from 16 fish. All isolates were identified using conventional and molecular methods. Then, they were examined for 5 different antibiotic resistance genes and their antibiotic susceptibility profiles were determined using the Kirby Bauer disc diffusion technique. Results of the disc diffusion test revealed that all 16 isolates had a different antibiotic susceptibility profile and the isolates with different antibiotic susceptibility profile could exist within and between farms, using the same water source. Furthermore, they revealed that highly resistant isolates that showed no susceptibility up to 82 to 100% of the 33 antibiotics at the doses evaluated exist in all farms. All isolates carried either tetA or tetB genes or both and the majority of isolates carried tetA gene. Together with this, 25% of the isolates which carried both or one of the resistance genes showed susceptibility to all 4 tetracycline class antibiotics at the doses evaluated. Results of the study and their comparison with previous studies in the same production area or in different regions of the country revealed the dynamic nature of antibiotic resistance development in L. garvieae. Additionally, it showed that monitoring studies with a limited number of isolates may not give an accurate picture of the current status of antibiotic resistance from a production area. These results of the study were also discussed in terms of the treatment strategies that trout farmers should follow when treating lactococcosis.

Disease outbreaks negatively affect fish production. Antimicrobial agents used in the treatment of diseases become ineffective over time because of antibiotic resistance developed by bacteria distributed in the aquaculture environment. This study was conducted for 4 months (cold period) in a fish farm to detect the fish disease, cold water streptococcosis. In the study, four brood stock showing disease signs were detected. Bacteria isolates were obtained and identified as Vagococcus salmoninarum. Antimicrobial susceptibility of V. salmoninarum was tested and antibiotic resistance gene profiles of V. salmoninarum isolates were screened. The phylogenetic relation of the isolates with the previously reported strains was evaluated. Antibiotic resistance developed by pathogenic bacteria is distributed in the aquaculture environment. The transfer of resistance genes from one bacterium to another is very common. This situation causes the antimicrobial agents used in the treatment of diseases to become ineffective over time. The disc diffusion test showed that all four isolates developed resistance to 13 (FFC30, AX25, C30, E15, CF30, L2, OX1, S10, T30, CRO30, CC2, PT15 and TY15) of the evaluated antibiotics and were about to develop resistance to six others (AM 10, FM 300, CFP75, SXT25, APR15 and TE30). Furthermore, antibiotic resistance genes tetA, sul1, sul2, sul3, dhfr1, ereB and floR were detected in the isolated strain. Moreover, the phylogenetic analysis showed that isolated V. salmoninarum strain (ESN1) was closely related to the bacterial strains isolated from USA and Jura.